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Old 31st July 2008, 08:54 PM   #1
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Default Transparent, Non-Microphonic Input Cap for Phono Preamp?

Hi,

I've recently noticed how microphonic large film caps can be when used in low-level / high-gain circuits such as phono preamps. I can't seem to find any in ratings below 200v at my preferred value, which means they are pretty large (I use 4u7 or 10uF). You see the 'scope waveform jump all over the place even if you lightly touch the cap :-( I dread to think what happens in MC stages if you crank up the speakers.

Aside from ridiculously expensive Teflon caps, what do you suggest for phono input caps? (Searching hasn't turned up too much info - I guess the terms 'input' and 'cap' are used too often).

Thanks in advance.

Justin
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Old 31st July 2008, 09:03 PM   #2
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I have used 10uF polycarbonate caps (Sprague) which are pretty good -- these are for timing and sample/hold circuits so D is very low -- they aren't particularly small, however. I also found that the Wima XLS 10.0 uF 16 V (MKS2-XL) which is polyester are also very good. I first saw these on a development board from Linear Tech and found a distributor here in NJ. There is virtually no distortion with the little WIMA's.

For tube stuff I use the big WIMA 4.7u/400V.

Are you sure that the microphonics are coming from the capacitor? -- I am aware that ceramics are not immune from this issue.
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Old 31st July 2008, 09:24 PM   #3
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Thanks for the leads.

I've gone around the circuit, tapping each component in turn. The only part that causes the 'scope to jump when tapped is the 200v / 4u7 polypropylene input cap.

A while ago I noticed a 1k tone in the workshop... I eventually traced it to a 630v / 4u7 cap that was at the front of the LTP in the amp I had on the bench - it was behaving like a little speaker!

I wonder if potting will improve things?

As an aside, my associates in the club sound arena complain about electrolytics being awfully microphonic (Nichicon Muse is favoured by these people). In 2006 I did a job at a large club in London, where I noticed that you'd get high-frequency howlround from the mixer with all the input channels muted!! I had to try it to believe it! I put it down to cheapo electrolytics blocking the outputs.

Justin
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Old 31st July 2008, 09:46 PM   #4
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Default Re: Transparent, Non-Microphonic Input Cap for Phono Preamp?

Quote:
Originally posted by thermionic
Hi,

I've recently noticed how microphonic large film caps can be when used in low-level / high-gain circuits such as phono preamps. I can't seem to find any in ratings below 200v at my preferred value, which means they are pretty large (I use 4u7 or 10uF). You see the 'scope waveform jump all over the place even if you lightly touch the cap :-( I dread to think what happens in MC stages if you crank up the speakers.

Aside from ridiculously expensive Teflon caps, what do you suggest for phono input caps? (Searching hasn't turned up too much info - I guess the terms 'input' and 'cap' are used too often).

Thanks in advance.

Justin
Hi thermionic

Why you chose a so big capacitor in the input? From the lot of plans that i have in my stock, in 99% of cases it is used a 100nF cap. I have recently builded a phono preamp (MM-MC) with succes by using this cap of 100nF. It is of first quality, brand SCR silver foil polypropylene thus MKP at 250V and has small dimensions. The big film caps, they presents also inductance because the many times rounded metal foil inside polypropylene.
Also phono cartridges haven't profitable response in very low frequencies due to rumble noise of turntables. You can see also in RIAA equalization curves, that is proposed a reduction in response by -3dB/oct below 60Hz (from the Philips AN142 for NE5534 this). Also in line level inputs, a 2,2F is enough big.

Fotios
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Old 31st July 2008, 09:50 PM   #5
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Default Re: Transparent, Non-Microphonic Input Cap for Phono Preamp?

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Originally posted by thermionic
Aside from ridiculously expensive Teflon caps, what do you suggest for phono input caps?

Is this a commercial design? There is simply no excuse for an input cap in a phono amp. You are probably right that teflons are the least microphonic but this is just no place for a cap.
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Old 31st July 2008, 09:55 PM   #6
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Default Re: Re: Transparent, Non-Microphonic Input Cap for Phono Preamp?

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Originally posted by fotios



Why you chose a so big capacitor in the input?

It will have to be large if it's a common base design.
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Old 31st July 2008, 10:03 PM   #7
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Default Re: Re: Transparent, Non-Microphonic Input Cap for Phono Preamp?

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Originally posted by analog_sa



Is this a commercial design? There is simply no excuse for an input cap in a phono amp. You are probably right that teflons are the least microphonic but this is just no place for a cap.
Input current offset (ammeter shorted to GND) is 4uA - so I definitely don't need an input cap. I can also match the LTP to minimise offset. However, there is a possibility that the design could be licensed to an OEM, and I don't think they will want to run the risk of a transistor blowing and magnetising an expensive cartridge. For my own setup I prefer to lose the cap.

Note that I didn't design the preamp. It is the work of pro designers that I know - I have been left holding the baby, i.e the subjective stuff. I realise I can get away with a smaller value cap, but I wanted to make sure that there would be no loss if someone used a MM cart with a particularly high output Z.

Justin
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Old 31st July 2008, 10:17 PM   #8
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Default Re: Re: Re: Transparent, Non-Microphonic Input Cap for Phono Preamp?

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Originally posted by thermionic


I realise I can get away with a smaller value cap, but I wanted to make sure that there would be no loss if someone used a MM cart with a particularly high output Z.


While i understand your predicament i don't understand the above. Presumably you can have a fairly high resistor biasing the ltp and a small cap in front.

Btw, i once saw a Threshold MC stage with a blown FET which had some nice power supply dc at input, so these situations do indeed happen.

Long time ago i used to use a LM394 based LTP. Worked fine with minimal input offset over many years.
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